The present invention relates to a dispensing instrument. The invention has been specifically designed for the dispensing of a semi-solid implantable material such as synthetic bone graft substitutes; demineralised bone matrices; bone graft extenders; calcium phosphate cements; polymethylmethacrylate cements; bone cements; dental cements. However, the dispenser may be used to dispense a variety of semi-solid materials or viscous liquids.
Known delivery systems for semi-solid materials in medicine have traditionally been based on two-part or three-part syringes where the dispensing of the material from the dispensing part is achieved by the user applied force transmitted directly through a plunger or through a separate displacement piston. This has limitations. For example, the maximum force generated by the user is relatively low due to the lack of mechanical advantage provided by the dispensers.
More complex systems for delivery of high viscosity liquids are known in the prior art.
One such product is the well-known mastic gun. This has a cradle-like portion arranged to receive a cartridge of high viscosity liquid, such as a filler or sealant. The device has a handle with a squeezable trigger. The trigger is pivotally mounted about an axis adjacent to the cartridge. One end of the trigger is squeezed by a user and the opposite end pushes on a piston within the cartridge. A spring biases the trigger so that, upon its release, it returns to its starting position allowing further movement of the trigger to selectively depress the piston and dispense more material. A spring loaded latch engages with a piston rod to prevent retraction of the piston rod during operation. When the cartridge is empty, the spring loaded latch is depressed and this allows the piston rod to be pulled out of the cartridge. In view of the cradle-like nature of the design, and also to ensure accurate dispensing of the material, the cartridge is provided with a nozzle through which the material is dispensed.
Such an arrangement is impractical for the dispensing of semi-solid implantable materials as the nozzle design generates a significant amount of latent hydraulic pressure in the liquid during dispensing which can cause an overrun of the material dispensed after the pressure on the handle has been relieved. The nozzle is also prone to clogging, particularly were it to be used with a semi-solid material. Similar devices are known in the medical field. A cement injection gun sold by Stryker®, and also by Orthovita® under the Cortoss® brand.